1. Field of the Invention
Apparatuses consistent with the present invention relate to a prism sheet and a backlight unit employing the same, and more particularly, to a prism sheet which can refract and transmit incident light, and to a backlight unit employing the same.
2. Description of the Related Art
In general, a backlight unit is used as a light source for a liquid crystal display (LCD) device having no self-illuminating capability and classified as a direct type backlight unit or an edge type backlight unit according to a position of the light source. In the direct type backlight unit, a lamp installed on a rear side of the LCD device emits light directly to a liquid crystal panel. In the edge type backlight unit, a lamp is installed at an edge of a light guide panel (LGP) and the light emitted by the lamp is guided by the LGP to proceed toward the liquid crystal panel.
FIG. 1 is a cross-sectional view of a conventional edge type backlight unit. Referring to FIG. 1, a cold cathode fluorescent lamp (CCFL) 1 is installed at an edge of an LGP 3. The LGP 3 has a tapered shape and a dispersion pattern 4 is formed on a lower surface of the LGP 3.
A reflection panel 5 is formed on the lower surface of the LGP 3 to reflect the light emitted from the CCFL 1 toward an LCD panel 20. Thus, the light emitted from the CCFL 1 is incident on the LGP 3 via an edge 3a. The incident light is changed to a surface light by the LGP 3 and the reflection panel 5 and proceeds toward an upper surface 3b of the LGP 3.
A diffusion panel 7 and a pair of prism sheets 10 are arranged on and above the upper surface 3b of the LGP 3. Each of the prism sheets 10 has a plurality of prisms arranged to neighbor one another. The prism sheets 10 include first and second prism sheets 10a and 10b which are divided according to the direction of arrangement and the role of the prisms. The prism arrangement directions of the first prism sheet 10a and the second prism sheet 10b are perpendicular to each other. The first prism sheet 10a corrects a vertical path, or a direction perpendicular to the plane of a drawing sheet, of the incident light having passed the LGP 3 and the diffusion panel 7, while the second prism sheet 10b corrects a horizontal path of the incident light. Thus, since the proceed paths of the incident light are corrected by the first and second prism sheets 10a and 10b, the corrected light can proceed toward the LCD panel 20.
A polarization panel 15 is provided between the prism sheet 10 and the LCD panel 20. The polarization panel 15 transmits light of a particular polarization only of incident light which has passed through the prism sheet 10, to proceed toward the LCD panel 20. Thus, the light exiting from the upper surface of the LGP 3 is diffused by the diffusion panel 7 and proceeds toward the LCD panel 20 with a path corrected by the prism sheets 10a and 10b. 
The prism sheet 10, as shown in FIG. 2, includes an incident portion 11 having an incident surface 11a and a plurality of prisms 13 formed on the opposite surface of the incident surface 11a, each having first and second exit surfaces 13a and 13b. The prism sheet 10 is disposed in air having a refractive index n0. The incident portion 11 and the prism 13 are integrally or separately formed of the same material having the same refractive index n. Thus, incident light L1 incident on the incident portion 11 is refractively incident on the incident portion 11 at an exit angle θ′ smaller than an incident angle θ. This is applied to all light incident on the incident portion 11. The light incident on the incident portion 11 linearly transmits at a boundary portion 12 between the incident portion 11 and the prism 13 without being refracted. In contrast, since the refractive index n of the prism 13 is higher than the refractive index n0 of the external air, the light incident on the first or second exit surfaces 13a and 13b exits at a refractive angle greater than the incident angle when the light is incident at an angle smaller than a critical angle θC, and the light incident at an angle greater than the critical angle θC is totally internally reflected.
Thus, while light L1, L2 and L3 of the light incident on the incident surface 11a are used as an effective light by exiting through the first or second exit surfaces 13a and 13b of the prism 13, lights L4 and L5 do not exit through the first or second exit surface 13a or 13b of the prism 13 and are totally internally reflected to proceed toward the incident surface 11a. Also, light L6 does not proceed toward the LCD panel 20 of FIG. 1 although it passed through the first and second exit surfaces 13a and 13b. Thus, since only a part of the light emitted from the light source 1 is used as the effective light, optical efficiency is deteriorated.
Further, since the backlight unit, configured as noted above, separately includes the light guide panel, the diffuser, and the prism sheet, the overall configuration is complicated and manufacturing costs rise.